I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for providing forward power control in a mobile communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," also assigned to the assignee of the present invention and is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user or mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM," and U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention and incorporated by reference herein.
If the transmission power of signals transmitted by the base station to a mobile station are too high, it can create problems such as interfering with other mobile stations. Alternatively, if the transmission power of signals transmitted by the base station are too low, then the mobile station can receive multiple erroneous frames. Terrestrial channel fading and other known factors can affect the transmission power of signals transmitted by the base station. As a result, each base station must rapidly and accurately adjust the transmission power of the signals which it transmits to the mobile stations.
In a useful method for controlling the transmission power of signals transmitted by a base station, the mobile station transmits a signal to the base station when a transmitted frame of data is received in error. In response to this message, the base station increases its transmission power for signals transmitted by the base station. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM" (the '109 patent), assigned to the assignee of the present invention and incorporated by reference herein.
A delay necessarily exists under the above CDMA closed loop communication system. The delay is the time between the time the base station transmits the frame with inadequate power and the time when, in response to an indication from the mobile station, the base station adjusts its transmission power. The delay includes the time when the base station transmits the frame with inadequate power, when the mobile station receives the erroneous frame, when the mobile station recognizes the error (e.g., a frame erasure), when the mobile station transmits the error message to the base station, and when the base station receives the error message and appropriately adjusts its transmission power. The CDMA communication system typically transmits packets of data as discrete frames. As a result, the CDMA communication system cannot rapidly compensate for transmission power fluctuations.
In order to optimize the transmission power of the base station, it is desirable for transmission power to be controlled by a centralized base station controller (BSC). However, centralized power control introduces an additional delay in the round trip path from the base station to the BSC, with the received information fed back, and from the BSC back to the base station. Therefore, a need is felt for a method to reduce the additional delay incurred by providing BSC power control while retaining the benefits of such a system.